In the 2008 movie The Dark Knight, Lucius Fox and Bruce Wayne have the following conversation in Hong Kong, prior to Batman capturing Lau (err, spoiler alert? :-)):

Bruce Wayne: What's this?
Lucius Fox: I had R&D work it up. Sends out a high-frequency pulse... records the response time for mapping an environment.
Bruce: Sonar. Just like a...
Lucius (interrupting with a smile): Submarine, Mr. Wayne. Like a submarine.

My question is: Why did Lucius interrupt Bruce and interject with "submarine"?

Was it just a subtle bit of humour to subvert the audience's expectations that they were going to compare it to bats using sonar to navigate? Or am I missing some other context (I'm not a reader of the comic books)?

For reference, one can watch the relevant clip here, starting at 5:10 (it's not my clip).